In recent years, such wireless instruments for receiving radio frequency signals with an antenna system embedded in the wristband have become common. Many prior art solutions disclose an antenna device having a circumferentially variable size, embedded in a wristband, for use with a radio that is worn on the arm of a person. By doing this, the antenna can be made long enough to receive frequency signals beyond the VHF band (30–300 MHz). As shown on FIG. 3, loop antenna 101 can be formed in a unitary fashion inside wristband 102, which is connected to casing 103 of wrist-carried wireless instrument 100 to form a continuous loop via a center fastening structure 104, for example a clasp, of the wristband when the band is fastened.
However, in such arrangements the loop connection at center fastening structure 104 significantly influences reception. Consequently it is difficult to design a mechanism that provides favourable operation, as this part is prone to break down. In addition, wristband 102 typically contains a wristband adjusting structure to adjust the length of the wristband to the thickness of the wearer's arm. This adjustment causes the antenna's loop length to vary from wearer to wearer, which causes variations in the receivable frequency band from one wearer to another.
A solution consisting in providing wireless instrument 100 with an additional apparatus for compensating changes in antenna gain and resonance frequency resulting from changes in the antenna's loop length, is complex and bulky, which is not desirable in such wireless instruments.
According to the U.S. Pat. No. 5,986,566, it is disclosed a solution, shown on FIG. 4, to prevent connection failure and/or breakdown due to attachment or detachment of a loop antenna, of the afore cited type, and to provide a wrist-carried wireless instrument whose receivable frequency band is not affected by the thickness of the wearer's arm.
Wrist-carried wireless instrument 110 includes a casing 113 and a center fastening-type wristband 112. Wristband 112 has upper 121 and lower 122 surfaces and a fastening structure 114 at its center and consists of a pair of wristband parts 112a and 112b, each of which is attached to an end of casing 113. A receiving antenna 111 is mounted inside in at least one part 112a of the wristband to receive signals, antenna 111 being connected via terminals to a known reception circuit inside casing 113. According to this document, loop antenna 111 extends between upper 121 and lower 122 surfaces of wristband 112 and does not go through center fastening structure 114. It is to be noted that reception would be possible without having wristband 112 attached and forming a loop, as it does when worn.
FIG. 5 shows a sectional view of a portion of wrist-carried wireless instrument 110 according to the prior art shown on FIG. 4. The same elements between FIGS. 4 and 5 are identified with the same numerical references. U-shaped antenna 111 is embedded in one part 112a of the wristband and is connected through casing 113 to an antenna receiver, not explicitly represented, located on a reception circuit substrate 114, via feeding lines for conveying received signals from the antenna to the antenna receiver. In this example the feeding lines are formed by terminals 115 soldered at the ends of U-shaped antenna 111 to provide connection with contact pins 116 who press on terminal springs 117 molded on substrate 114.
In such small antennas, the radiation resistance is very small compared to ohmic and dielectric or permeability antenna losses caused by electric conductors, dielectric or magnetic materials used in the wireless instrument. Therefore, the antenna gain is predominantly given by antenna losses. Because loss of the antenna compared to radiation resistance is very high, the loop antenna geometry has to be carefully chosen with a maximum radiating surface and minimum antenna losses.
Nevertheless within the scope of the present invention, measures done on the antenna structure according to the U.S. Pat. No. 5,986,566 have shown up non-optimum antenna efficiency due to non-negligible losses. As a matter of fact, the antenna radiating element of the antenna structure, as shown on FIG. 5, includes not only U-shaped loop 111 inside the wristband part but also feeding lines 115, 116 and 117 connecting the loop antenna to the antenna receiver inside casing 113. Furthermore, when wireless instrument 110 is worn on the user's arm, the U-shaped loop and the feeding lines are nearly right-angled as shown on FIG. 6. Resulting radiating surface RS1+RS2 of the antenna radiating element (U-shaped loop and feeding lines), referenced B as a whole, is in a plane Ph parallel to hypotenuse h of the right triangle formed by the U-shaped loop and the feeding lines and corresponds to the sum of both radiating surface projections RS1 and RS2 related to the contribution of each part of the radiating element in the aforementioned plane Ph. Thus, although resulting radiating surface RS1+RS2 increases slightly, in the meantime antenna losses increase significantly because they depend on the antenna inductance which increases with the total length of the radiating element, and then overall antenna efficiency is significantly reduced.
Alternative solutions that would consist in replacing the U-shaped antenna with a multi-loop antenna, is not desirable because manufacturing process of such multi-turn antennas is more difficult.
It is then an object of the present invention, to optimise geometry of the wristband embedded antenna to obtain a good compromise between the size of the radiating surface and antenna losses.